The invention relates to an integrated transistor circuit in MOS-technology, constructed to form a variable resistor between a first terminal and a second terminal and comprising
a first and a second transistor each having a control electrode and a main current path between a first and a second main electrode, the first and the second main electrode of the first transistor being coupled to the first and the second terminal respectively and the first and the second main electrode of the second transistor being coupled to the second and the first terminal respectively, PA0 a third and a fourth transistor each having a control electrode and a main current path between a first and a second main electrode, and PA0 a first and a second controllable current source coupled to the main current path of the third and the fourth transistor respectively. PA0 To this end an integrated MOS transistor circuit in accordance with the invention is characterized in that PA0 the control electrode of the third transistor is coupled to the second main electrode of the third transistor and to the control electrode of the first transistor, PA0 the control electrode of the fourth transistor is coupled to the second main electrode of the fourth transistor and to the control electrode of the second transistor, PA0 the first main electrode of the first transistor is coupled to the first main electrode of the third transistor, and PA0 the first main electrode of the second transistor is coupled to the first main electrode of the fourth transistor.
Such a circuit is disclosed in P. E. Allen and D. R. Holberg: "CMOS Analog Circuit Design", page 217, FIG. 5.2-5, Holt, Rinehart and Winston. In integrated MOS circuits it is common practice to employ active MOS transistors as resistors instead of passive polysilicon or diffused passive resistors. Generally, active resistors not only have a smaller surface area than equivalent passive resistors but they are also adjustable. In said active resistors the channel of the main current path between the main electrodes (source, drain) of the MOS transistor functions as the resistor. The resistance of the channel is controlled by means of a variable voltage between the control electrode (gate) and the first main electrode (source) of the MOS transistor. However, the channel resistance of the main current path depends not only on the selected gate-source voltage but also on the value and polarity of the drain-source voltage. The channel resistance of the main current path is substantially constant over a specific range of drain-source voltage, so that it may be referred to as a voltage-independent linear resistance. However, as the drain-source voltage increases the non-linear relationship inherent in MOS transistors, between drain-source current in and drain-source voltage across the main current source becomes manifest. As a result of this the drain-source voltage range within which the channel resistance of a MOS transistor remains fairly constant is rather limited. A further limitation is imposed by what is referred to as the "body effect" or "bulk effect", sometimes also "backgate effect", of an integrated MOS transistor. This effect manifests itself if the voltage difference between the substrate of the integrated circuit and the first main electrode (source) of the main current path of an integrated MOS transistor is not constant. This is so in particular if the MOS transistor is arranged as a resistor which is floating relative to the substrate. The substrate then behaves as an additional control electrode (backgate), which adversely affects the channel resistance of the main current path. The body effect also has a non-linear influence on the channel resistance.
Said prior-art transistor circuit provides a solution to arrive at an extended voltage range within which the main current path of a floating MOS transistor arranged as an active resistor can be used as a linear resistor. In order to form an active controllable resistor the main current path of a first transistor and a second transistor are arranged in parallel between a first and a second terminal. The bias voltages on the control electrodes of the first and the second transistor are respectively furnished by a first controllable current source coupled to the main current path of a third transistor and by a second controllable current source coupled to the main current path of a fourth transistor. In this prior-art configuration the non-linear effects on the channel resistance caused by the non-linear behavior inherent in an integrated transistor are compensated for by cross-coupling the main electrodes of the first and the second transistor. The non-linear effects then cancel one another, resulting in a substantial extension of the range of drain-source voltages within which the resistor thus formed is linear. Although the above-mentioned book states that cross-coupling also results in the body effect in the first transistor being cancelled by the body effect in the second transistor, this is not or not entirely true. In the prior-art circuit the instantaneous voltages across the main current paths of the first and the second transistor still influence the backgate-source voltages of these transistors.